This invention relates to a semiconductor device, particularly a semiconductor device comprising a semiconductor element having the surface coated with an .alpha.-ray-insulating polyimide, and to a method for production of such a semiconductor device.
Semiconductor devices generally have a structure such that a semiconductor element is secured onto a support made of, for example, a ceramic or Kovar (an Fe-Ni-Co alloy) and this semiconductor element is sealed by wall and lid members made of, for example, a ceramic.
When the semiconductor element to be sealed is constructed by a high density integrated circuit, especially an MOS device or charge transfer device, degradation of characteristics, such as destruction of stored information, is caused by radiations from a surrounding structural member or sealing member, especially .alpha.-rays. Occurrence of this undesirable phenomenon is due to the fact that uranium (U) and thorium (Th), which are present in the natural world and produce .alpha.-rays as the result of radioactive decay, are contained in the ceramic material constituting the surrounding member, in a low-melting-point glass as the sealing member or in a solder comprised of lead and tin.
When .alpha.-rays thus produced intrude into the semiconductor element, pairs consisting of a positive hole and an electron, are generated and either the positive hole or the electron of each pair is injected into the active region of the semiconductor element to cause, for example, the above-mentioned destruction of stored information. Accordingly, it is important to prevent irradiation and intrusion of .alpha.-rays in the surface region of a semiconductor substrate.
The inventors have found that the undesirable intrusion of .alpha.-rays into a semiconductor element can be prevented or minimized by coating the surface of the semiconductor element with a polyimide film for blocking intrusion of the .alpha.-rays. In order for this polyimide film to exert this .alpha.-ray-blocking effect, the polyimide film should have a thickness of approximately 20 to 30 .mu.m. When a spin coating method is adopted for formation of a polyimide film, if the speed of rotation is small, such a thickness can be obtained by performing the coating operation only once.
However, when the speed of rotation is small, as it is in this case, the thickness distribution in the wafer becomes non-uniform, with the result that a semiconductor element obtained from one wafer fails to have a uniform thickness of 20 to 30 .mu.m. If the rotation speed is elevated to a level sufficient to make the thickness uniform, the thickness of the polyimide film formed by performing the coating operation only once is about 10 to about 15 .mu.m at the largest. Therefore, in order to obtain a film having a thickness of 20 to 30 .mu.m, it is necessary to conduct the coating operation at least two times. However, since the solvent used must be removed by a heat treatment after every coating operation, if the coating operation is repeated two times or more, the number of steps is inevitably increased.
Furthermore, in order to obtain a final product with the semiconductor element having such a polyimide film, a window for a bonding pad should be formed on the polyimide film to wire-connect the semiconductor element to an external circuit. For formation of such a window, an etching treatment is carried out by using, for example, a highly alkaline etchant containing hydrozine and ethylenediamine. A mask is necessary for this etching treatment, and in order to prevent the etchant from corroding the metal of the bonding pad, it is necessary to adopt a method in which a protecting film, comprised of, for example, phosphosilicate glass (PSG), is formed and etched in a controlled manner to leave a thin film in the pad portion and, in the succeeding step, the PSG film in the pad portion is removed leaving the polyimide, which has been formed in a separate step, as a mask by using another etchant.
The above-mentioned process for manufacturing a semiconductor device having a protective polyimide film layer will be illustrated by FIGS. 1A, 1B and 1C. Referring to FIG. 1A, a PSG film 3 is coated on a metal layer 2 of aluminum or the like formed as a bonding pad on a semiconductor substrate 1 on which a semiconductor element is formed, and then, the pad portion is etched in a controlled manner and thereafter a polyimide is coated thereon to form a polyimide film 4. Then, as shown in FIG. 1B, the polyimide film 4 is etched with a highly alkaline etchant by using a mask (not shown) to open a window 5. Thereafter, as shown in FIG. 1C, a window is formed in the PSG film 3 by a fluoric acid type etchant with the polyimide film 4 being as the mask. As is seen from the above description, a mask should be used for formation of a window on the polyimide film and the etching treatment should be conducted two times by using different etchants, with the result that the process becomes complicated.
As a means for eliminating the above-mentioned disadvantages, there is a method in which the screen printing technique is adopted.
This method is ordinarily utilized for patterning of wires in hybrid integrated circuits. However, if a layer of polyimide is coated in a thickness of 20 to 30 .mu.m, since the viscosity of the polyimide is low, sagging of the pattern is readily caused. In the case where this polyimide undesirably covers the bonding pad portion, if the property test is carried out by using a test probe brought into contact with the bonding pad, the product may be judged as a defective product because of insufficient contact of the bonding pad though the formed semiconductor element. Furthermore, in this case, insufficient bonding is readily caused.